1. Field of the Invention
The present invention relates to an optical scanning apparatus for detecting an object by scanning a laser beam, particularly to an optical scanning apparatus for making the width of an emitted laser beam (beam width) variable in accordance with the situation of an object present in a forward direction.
2. Description of the Related Art
There has conventionally been known an optical scanning apparatus for detecting an object present in a forward direction by a laser beam. According to such an optical scanning apparatus, there is generally adopted a system in which a mirror is arranged in front of a light emitting element of, for example, a laser diode (LD) or a light emitting diode (LED), where a direction of the beam emitted from a light source is changed by the mirror and an object is detected by scanning the beam.
Accordingly, a polygonal mirror is used for changing the direction of the emitted laser beam. A beam emitted from a laser oscillator is made incident on the polygonal mirror and the direction of the emitted laser beam is changed by reflecting the laser beam by the polygonal mirror. Such a system is disclosed in Japanese Patent Laid-Open No. 115912/1985, Japanese Patent Laid-Open No. 8119/1987 and Japanese Patent Laid-Open No. 45600/1993.
According to the system disclosed in the above-described publications, there is provided a laser oscillator having a laser diode (light source) and a polygonal mirror for deflecting an incident beam. A laser beam emitted from the laser oscillator is made incident on the rotating polygonal mirror and reflected laser beam is emitted to outside. In this case, in order to change a state of deflecting the laser beam, angles of inclination of reflecting faces formed in the polygonal mirror are changed for respective faces and the polygonal mirror is driven to rotate by a motor to thereby change a direction of emitting the laser beam which is made incident on the polygonal mirror and reflected thereby and deflect the emitting direction two-dimensionally in a constant shape of the beam.
Specifically, as shown in FIG. 15, a polygonal mirror having six faces is provided with reflecting face angles (.theta.1, .theta.2, . . . .theta.6) respectively in its reflecting faces. By rotating the polygonal mirror in the direction of the arrow by a drive apparatus such as a motor or the like, the incident direction of an incident laser beam reflected by a reflecting plate, is changed by the rotation. For example, when a laser beam is emitted in a predetermined light emitting pattern and is made incident on the polygonal mirror, the polygonal mirror is rotated with respect to one reflecting face having an angle of inclination of .theta.1 as shown in FIG. 15. A position irradiated with the laser beam is scanned from left to right and the scanning is executed by one line in the horizontal direction. Thereafter, when a position on which the laser beam is incident is shifted to a successive reflecting face having an angle of inclination of .theta.2 by rotating the polygonal mirror, the scanning line is changed in the vertical direction and a second scanning is carried out. By similarly carrying out scanning of a third through a sixth line, two-dimensional scanning is carried out in a predetermined region.
When an object in a forward direction is detected by laser beam or a distance to the object is measured, an optical intensity and a measured distance are defined generally by a radar equation, shown below. ##EQU1##
where Prtn: intensity of reflected beam (W). Pout: peak output of light source (W), d: measured distance (m), .alpha.O: transmittance of light transmitting system, .alpha.r: light transmittance of light receiving system, K: reflectivity of targeted object, .theta..sub.1 : transmitted beam horizontal divergence angle (rad), .theta..sub.2 : transmitted beam vertical divergence angle (rad), .phi.: divergence angle of reflected beam (rad), Aref: area of target object (m.sup.2), Ar: received portion opening area (m.sup.2), T: transmittance of atmospheric air (changed by meteorological condition).
According to the radar equation, when light intensity (light source peak output) is fixed, the measured distance d is changed by a shape .theta.1, .theta.2 of emitted beam. For example, when the light source peak output is fixed and irradiation angles (beam angles) of the laser beam in the horizontal and vertical directions are made as proximate to those of a parallel beam as possible (beam is narrowed), the measured distance d becomes long and when the beam angles are diverged (widened), the measured distance is shortened.
When an apparatus for emitting a laser beam is applied to an inter-vehicle distance control based on the above-described feature, there is adopted a method of scanning a wide area by narrowed beam. In this case, laser beam is irradiated with a remote object (here, a foregoing vehicle) as a target and accordingly, a scanning area (detecting area) is narrowed in a very short distance. Therefore, when an interrupting vehicle enters within a short distance from the horizontal direction, the detection is retarded, control is retarded when an inter-vehicle distance between a foregoing vehicle and a driving vehicle is calculated from information based on reflected beam of laser beam and speed control of the driving vehicle is carried out based thereon and accordingly, a desired function cannot be achieved sufficiently.
In the case of carrying out follow-up running, when an optical scanning apparatus is used as a sensor for detecting a foregoing vehicle, laser beam is emitted with a reflector at a rear face of a vehicle having high optical reflectivity as a target. Generally, as prescribed in Safety Code Article 38, Paragraph 5 the height for attaching the reflector is prescribed to be equal to or larger than 0.25 m and equal to or smaller than 1.5 m and in the case in which a road is provided with a maximum road width of 3.5 m, when there is a foregoing vehicle at a very short distance (for example, 10 m or smaller), there is produced an area which cannot detect the reflector by the laser beam. Accordingly, only with beam by the current scanning operation, there causes a failure in detection in respect of a target (foregoing vehicle) at a very short distance (for example, about 10 in through 30 m) and a countermeasure there against is needed.